Crime scene investigators are often challenged with the task of detecting and analyzing forensic evidence that is not visible to the human eye, most notably latent fingerprints. To detect otherwise invisible forensic evidence, including latent fingerprints, investigators have relied on numerous techniques utilizing various wavelengths of light. Some systems have relied upon wavelengths that are included in the ultraviolet (“UV”) spectrum, also known as UV radiation. Latent fingerprint detection techniques have historically required additional fluorescent chemical processing, such as dusting with a fluorescent dusting powder or chemical dyeing prior to UV radiation exposure, in order to make the fingerprints visible to the unaided eye or otherwise capable of being captured by an image capturing device or other capturing techniques. These dusting and dying techniques may be disadvantageous, however, at least because once they are applied, they may prevent investigators from extracting DNA evidence that may also be included in the forensic sample.
In certain situations, RUVIS (Reflective Ultra-Violet Imaging System) technology, which enables a user to see latent fingerprints on nonporous surfaces without requiring treatment with powders or dyes, may be used to examine latent fingerprints. RUVIS technology works on the principle that surfaces either reflect or absorb light to varying degrees in the UV spectrum. When UV radiation illuminates a surface containing a latent fingerprint, the fingerprint stands out as darker or lighter than the background based on whether the surface reflects or absorbs the UV radiation more or less than the latent fingerprint, respectively. In many situations though, a latent fingerprint may be provided on a surface that makes it difficult to accurately capture the fingerprint.
For example, in certain situations, it has been difficult and time consuming to photograph, digitally or otherwise, latent fingerprints or other evidence deposited on a curved or irregular surface such as a wine glass, shell casing, pen, or crinkled aluminum foil. The nature of the curved surface may also contribute to difficulty in capturing a latent fingerprint because a light source used on a reflective surface may create a glare (specular reflection), which can also complicate the capture of contrasted aspects of a latent fingerprint due to illumination of overly bright aspects of the fingerprint. Traditionally, in these situations, the latent fingerprint was either chemically treated so as to fluoresce, or it was necessary to evenly illuminate the latent fingerprint using multiple light sources to minimize the specular reflection. Even illumination of a subject using multiple light sources may be difficult to attain, however, and may still result in aspects of the fingerprint being overly illuminated thus degrading the quality of the captured image.
Additionally, in these situations, because certain aspects of the fingerprint are farther away from the camera lens, such as the edges of a fingerprint on a curved surface, a lens with a small aperture may be employed to maximize the depth of field and maintain focus over the curved surface. Because of the small aperture, the camera exposure time must then be increased in order to gather enough light to capture an image of the fingerprint. But, with some shiny surfaces containing latent fingerprints, such as aluminum cans and metal shell casings, it may be difficult to avoid specular reflection from the one or more light sources. And when using a longer exposure, some aspects of the captured image may be overexposed due to the overly bright specular reflection, thus making it very difficult to capture a latent fingerprint of sufficient quality.
In addition, it has been difficult or impossible to photograph chemically treated (or untreated), latent fingerprints (chamber marks or other evidence) deposited on a small diameter curved surface, such as a small caliber shell casing. Due to the small diameter, the edges of a latent fingerprint, for example, may be significantly farther away from a camera lens as compared to a center of the fingerprint closest to a camera lens. Due to the physical limitations of optical devices, the entire latent fingerprint may not be seen in focus at one time from any angle.
Thus, some systems have relied upon taking multiple images at different angles to capture a fully focused fingerprint. The multiple images were then stitched together electronically. Such stitching methods, however, require considerable time and skill to correctly append aspects of the various images along precise boundaries of the fingerprint to recreate the fingerprint in a single image. Because these methods may require considerable user interaction, there may be an increased risk of user error that reduces the reliability or authenticity of an image of a fingerprint specimen generated in this manner. Stitching numerous images together in this manner may also create a very large image-file-size that may be difficult to work with in a number of other applications.